Oxidation resistant superalloy

ABSTRACT

A nickel-based superalloy that forms a chromia scale in an oxidizing environment is disclosed. The alloy provides good oxidation resistance at temperatures below 900° C. in a dry or moist atmosphere. The superalloy is well-suited for components of gas or steam turbine engines including blades and vanes.

FIELD OF THE INVENTION

The invention relates to a nickel-based superalloy that forms a protective chromia layer in an oxidizing atmosphere.

BACKGROUND OF THE INVENTION

Nickel-based superalloys have a very good material strength at elevated temperatures. These properties permit their use in components for gas turbine engines where the retention of excellent mechanical properties at high temperatures is required for typical lifetimes in excess of 100,000 hours. Components within an industrial gas turbine are exposed to a range of temperatures depending upon where the component is located in the turbine. For example, last stage turbine blades see intermediate temperatures and do not require and are not covered by a thermal barrier coating. The last stage turbine blades are protected by the surface oxide layer that forms upon exposure to the oxidizing environment.

The metallurgy of superalloys is a sophisticated and well developed field. Optimization of the composition of superalloys consists of defining the amounts of elements which are desirably present, and the amounts of elements which are desirably absent. These impurities can in some cases be completely eliminated from the composition through the judicious selection of melt stock material; however, some elements cannot be readily eliminated. One impurity which has long been recognized as being detrimental is sulfur. Sulfur was initially identified as being detrimental to mechanical properties, and its presence in alloy compositions was limited for that reason. However, the sulfur levels which do not present significant loss of mechanical properties, a bulk property, can in some cases still be highly detrimental to oxidation resistance, a surface property.

Oxidation resistance of superalloys is primarily due to the presence of an adherent surface oxide scale. The composition and nature of oxide scales depends on the composition of the alloy and the environment in which the superalloy component operates. Typically, the oxide is either alumina or chromia. These oxides are formed by a selective oxidation of the component in the alloy. A continuous oxide scale of predominately or exclusively one metal occurs where a metal at or above a critical concentration in the alloy oxidizes preferentially over more noble metals, such that the sufficiently high concentration less noble element diffuses to and into the scale, forming more of the selective oxide while the more noble metal diffuses from the scale into the alloy. The composition and thickness of the protective oxide scale formed depends on a number of factors including the relative concentration of the metals in the alloy, the relative nobilities of the metals in the substrate, and oxygen solubility and diffusivity in the alloy. As the temperature changes, the diffusion rates and solubilities of the metals and oxygen will change. Typically, the formation of an alumina scale is favored by higher temperatures. Some alloys form chromia scales at lower temperatures and alumina scales at higher temperatures where aluminum no longer precipitates internally. Generally maximum protection via alumina occurs by formation at a temperature range, in excess of 1,000° C. The selective oxidation can also be enhanced by the incorporation of reactive metals such as rare earth elements.

Moisture affects the degradation and debonding of the protective oxide. This is particularly critical for steam turbines, where superalloys are replacing steel as the temperature at which the turbine operates increases. The temperatures at which a steam turbine operates are typically lower than the temperatures where gas turbine operates. The presence of water vapor generally lowers the oxidation stability of the superalloy, as the oxide scale is either poorly formed, or is not stable to the water vapor containing oxidizing environment. Selective oxidation to alumina is not favored in a water containing atmosphere, particularly at lower temperatures. At higher temperatures, in excess of 900° C., water vapor can promote the formation of volatile species that remove chromia scale and can ultimately result in the loss of oxidation protection by a chromia scale.

Hence the identification of superalloys that provides good oxidation resistance to components, particularly for components that must operate in the presence of water vapor.

SUMMARY OF THE INVENTION

This invention is directed to a nickel-based superalloy that forms a highly adherent chromia surface layer when exposed to an oxidizing environment at intermediate elevated temperatures. The nickel-based super alloy may be used together with industrial gas turbine components operating at intermediate temperatures, such as last stage turbine blades, which are not protected by a coating. The life of such components may be greatly extended by forming an outer layer of alumina or chromia.

In one embodiment, the nickel-based superalloy may be formed from materials in the following weight percentages: 15.5 to 18.5 Cr; 14.0 to 15.5 Co; 2.5 to 3.5 Mo; 2.0 to 3.5 Al; 4.0 to 5.5 Ti; 0.5 to 2.0 W; 0 to 1.5 Hf; 0 to 2.0 Fe; 0 to 1.0 Si; 0.01 to 0.1 B; 0 to 0.1 Zr; 0.03 to 0.20 C; 0 to 0.5 at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; less than 0.1 S; and the balance formed from Ni. A preferred superalloy may be formed from materials in the following weight percentages: 15.5 to 16.5 Cr; 14.0 to 15.5 Co; 2.75 to 3.25 Mo; 2.25 to 2.75 Al; 4.75 to 5.25 Ti; 1.0 to 1.5 W; 0 to 1.0 Hf; 0 to 0.5 Fe; 0 to 0.5 Si; 0.01 to 0.02 B; 0.025 to 0.08 Zr; 0.03 to 0.10 C; 0 to 0.2 at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; less than 10 ppm S; and the balance formed from Ni. A more preferred superalloy may be formed from materials in the following weight percentages: 15.5 to 16.5 Cr; 14.25 to 14.75 Co; 2.80 to 3.20 Mo; 2.30 to 2.60 Al; 4.80 to 5.20 Ti; 1.2 to 1.5 W; 0.10 to 0.15 Hf; 0 to 0.2 Fe; 0.10 to 0.15 Si; 0.01 to 0.02 B; 0.025 to 0.050 Zr; 0.03 to 0.04 C; 250 to 750 ppm at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; less than 2 ppm S; and the balance formed from Ni.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to a nickel-based superalloy that forms a highly adherent chromia surface layer when exposed to an oxidizing environment at intermediate elevated temperatures. The nickel-based super alloy may be used together with industrial gas turbine components operating at intermediate temperatures, such as last stage turbine blades, which are not protected by a coating. The life of such components may be greatly extended by forming an outer layer of alumina or chromia.

In one embodiment, the superalloy can form and maintain a well adhered protective chromia scale for use at intermediate temperatures, which provides oxidation resistance when exposed to a dry or moist gas and is suitable for components used in a gas or steam turbine engine. The superalloy may be formed from materials in the following weight percentages: 15.5 to 18.5 Cr; 14.0 to 15.5 Co; 2.5 to 3.5 Mo; 2.0 to 3.5 Al; 4.0 to 5.5 Ti; 0.5 to 2.0 W; 0 to 1.5 Hf; 0 to 2.0 Fe; 0 to 1.0 Si; 0.01 to 0.1 B; 0 to 0.1 Zr; 0.03 to 0.20 C; 0 to 0.5 at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; less than 0.1 S; and the balance formed from Ni.

A preferred superalloy may be formed from materials in the following weight percentages: 15.5 to 16.5 Cr; 14.0 to 15.5 Co; 2.75 to 3.25 Mo; 2.25 to 2.75 Al; 4.75 to 5.25 Ti; 1.0 to 1.5 W; 0 to 1.0 Hf; 0 to 0.5 Fe; 0 to 0.5 Si; 0.01 to 0.02 B; 0.025 to 0.08 Zr; 0.03 to 0.10 C; 0 to 0.2 at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; less than 10 ppm S; and the balance formed from Ni. A more preferred superalloy composition may be formed from materials in the following weight percentages: 15.5 to 16.5 Cr; 14.25 to 14.75 Co; 2.80 to 3.20 Mo; 2.30 to 2.60 Al; 4.80 to 5.20 Ti; 1.2 to 1.5 W; 0.10 to 0.15 Hf; 0 to 0.2 Fe; 0.10 to 0.15 Si; 0.01 to 0.02 B; 0.025 to 0.050 Zr; 0.03 to 0.04 C; 250 to 750 ppm at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; less than 2 ppm S; and the balance formed from Ni.

The superalloy of the present invention is intended to be used for components where a chromia scale provides oxidation resistance. It is also intended that components produced from this superalloy be used at intermediate temperatures generally in the range of between 450° C. to 750° C. and are not intended for service at temperatures of above 900° C. The turbine components prepared from the inventive superalloy can be used in the presence of a dry gas or with a gas that includes water vapor. Therefore, the components from the inventive superalloy can be used in either a gas or steam turbine engine.

The inventive superalloy may have a chromium (Cr) content between 15.5 and 18.5 weight percent. This level of Cr supports the formation of a chromia scale with little or no other metal oxides included in the scale. To assure an excellent well adhered chromia scale, the scale should be almost exclusively chromia with little content of other metals. The preferred level is 15.5 to 16.5 weight percent Cr, which assures that a well adhered chromia scale forms.

Aluminum (Al) is included in the superalloy at levels of 2.0 to 3.5 weight percent. At the intermediate temperatures for use of the inventive superalloy, the level of Al is insufficient to form an alumina scale rather than remain primarily as an alloy element in the gamma prime phase.

Titanium (Ti) is included at 4.0 to 5.5 weight percent in the inventive alloy and, in general, will reside in the gamma prime phase of the superalloy where it acts as a solid-solute strengthener. In most cases, titania will not be present in the chromia scale. However, some titania can be included in the chromia scale when the scale is formed near the upper temperature limits for use of the inventive superalloy. The titania can reside at the gas/chromia interface and act as a physical barrier to the loss of volatile chromium oxide species.

Sulfur (S) is preferably absent from the superalloy, but is generally present as an impurity in the superalloy. Spalling of the oxide scale is promoted by S and for this reason S must be bound into the superalloy or be present at a very low level, and is preferably below 10 ppm.

One or more rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd can be included in the inventive superalloy. The inclusion of the rare earths aids in the formation, adherence, and maintenance of the chromia scale. The rare earth elements also selectively combine with sulfur to form refractory sulfides in the superalloy, preventing sulfur migration to the scale where it is detrimental to chromia adhesion to the superalloy.

Cobalt (Co) replaces nickel (Ni) in the gamma-phase to strengthen the matrix in solid solution. Co is included in the range of 14.0 to 15.5 percent by weight in the present invention to strengthen the matrix in solid solution. A preferred range for Co is from 8.5 to 9.5 percent by weight.

Tungsten (W) is a solid-solute strengthener of the gamma-phase. In the present invention, a W content is included at 0.5 to 2.0 weight percent and is preferably 1.0 to 1.5 weight percent.

Alternatives for the alloy composition and other variations within the range provided will be apparent to those skilled in the art. Variations and modifications can be made without departing from the scope and spirit of the invention as defined by the following claims. 

1. A nickel-based superalloy expressed in weight percentages consisting essentially of: 15.5 to 18.5 Cr; 14.0 to 15.5 Co; 2.5 to 3.5 Mo; 2.0 to 3.5 Al; 4.0 to 5.5 Ti; 0.5 to 2.0 W; 0 to 1.5 Hf; 0 to 2.0 Fe; 0 to 1.0 Si; 0.01 to 0.1 B; 0 to 0.1 Zr; 0.03 to 0.20 C; 0 to 0.5 at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; less than 0.1 S; and balance formed from Ni.
 2. The superalloy of claim 1, wherein the superalloy expressed in weight percentages consisting essentially of: 15.5 to 16.5 Cr; 14.0 to 15.5 Co; 2.75 to 3.25 Mo; 2.25 to 2.75 Al; 4.75 to 5.25 Ti; 1.0 to 1.5 W; 0 to 1.0 Hf; 0 to 0.5 Fe; 0 to 0.5 Si; 0.01 to 0.02 B; 0.025 to 0.08 Zr; 0.03 to 0.10 C; 0 to 0.2 at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; less than 10 ppm S; and the balance formed from Ni.
 3. The superalloy of claim 1, wherein the superalloy expressed in weight percentages consisting essentially of: 15.5 to 16.5 Cr; 14.25 to 14.75 Co; 2.80 to 3.20 Mo; 2.30 to 2.60 Al; 4.80 to 5.20 Ti; 1.2 to 1.5 W; 0.10 to 0.15 Hf; O to 0.2 Fe; 0.10 to 0.15 Si; 0.01 to 0.02 B; 0.025 to 0.050 Zr; 0.03 to 0.04 C; 250 to 750 ppm at least one rare earth elements selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; less than 2 ppm S; and the balance formed from Ni. 